Systems, methods, and measuring devices by which an analyte, and more specifically electrolytes such as, for example, potassium, can be selectively extracted and measured easily and in a minimally invasive manner using a small amount of biological fluid, such as blood, plasma, or serum. The measuring device or chemical sensor is a test strip including a layered active component assembly sandwiched between two optional film or support layers. The layered active component assembly includes ion-sensitive membranes for performing ion-selective extraction methods. The test strips can be utilized with an optical reader connected to a smartphone or other mobile device for remote monitoring of an analyte level associated with conditions of a disease.